In recent years, there was an increase in demand for smaller size and lighter weight spindle motors. There was also an increase in demand for higher density of memory capacity in data recording devices such as magnetic disks and optical disks used in computers. These developments led to an increased demand for technologies increasing motors' rpm speed and improving rotation accuracies in spindle motors used to rotate such disks.
To address this demand with respect to bearings used to support rotating shafts in data storage devices, there has been an increasing trend away from conventional ball bearings toward the adoption of fluid dynamic pressure bearings. Fluid dynamic pressure bearings support a rotating shaft by generating a fluid dynamic pressure within lubricating fluid, for example oil or air, when the shaft is rotated.
Fluid dynamic pressure bearings are well known in the art. Structures which employ fluid dynamic pressure bearings as bearings for spindle motor rotating shafts are also well known (see, for example, Japanese Patent No. 2937833). An example of a conventionally known fluid dynamic pressure bearing is shown as the conventional example in FIGS. 4(a)-(c).
As shown in FIGS. 4(a)-(c), a rotating shaft 31 is supported for rotation inside a bearing sleeve 32, thus defining the bearing portion. Lubricating oil is enclosed in the gap formed between the inner perimeter surface and bottom surface of bearing sleeve 32 and the outer surface of the rotating shaft. A radial fluid dynamic pressure-generating groove 33 is formed on the inner perimeter surface of the bearing portion of the bearing sleeve, while a thrust fluid dynamic pressure-generating groove 34 is formed on the bottom surface of bearing sleeve 32.
When shaft 31 rotates, the fluid dynamic pressure generated by radial fluid dynamic pressure-generating groove 33 and thrust fluid dynamic pressure-generating groove 34, in the radial and thrust directions, respectively, enables the rotating shaft to rotate in a suspended state inside the bearing sleeve, with a film of lubricating oil interposed therebetween.
During operation of the above described spindle motor, lubricating oil 12 enclosed in the gap 35 between rotating shaft 31 and sleeve 32 ascends to the opening surface at the top edge portion of sleeve 32. This oil ascending phenomenon may be caused by volumetric changes from temperature change-induced expansion and contraction of the lubricating oil, expansion displacement of the bearing dimensions, internal movement caused by the pumping effect at the start and stop of shaft's rotation or effects of centrifugal forces and dynamic pressure during rotation.
This type of ascending of the lubricating oil such that it reaches and overflows the opening surface of the bearing sleeve creates the problem of lubricating oil leakage. Leakage and depletion of the lubricating oil from the bearing sleeve results in insufficient fluid dynamic pressure, reduced lubrication, and, in some cases, burning through contact between the rotating shaft and the bearing sleeve. At the same time leaking lubricating oil can erase recordings on the magnetic disk.
As shown in FIGS. 4(a)-(c), a gap widening portion 37 having a tapered surface 36 may be provided at the upper portion of the bearing sleeve to prevent leakage of lubricating oil. Gap widening portion 37 gradually expands at a specified angle of inclination a, as measured between the inner surface of bearing sleeve 32 and the axis of the shaft at the gap opening edge area. Thus, the upper portion of the gap gradually widens in the direction of the opening surface. Further, as shown in FIG. 4(c), the bearing may also include a lubricating oil reservoir 38 disposed on the inner surface of bearing sleeve 32, specifically, on the inside of tapered surface 36.
As disclosed in the above-mentioned Japanese Patent No. 2937833, an oil collecting groove may be disposed on the inner surface of the bearing sleeve. A gap changing portion is also provided in the disclosed construction, wherein the gap expands towards the opening surface of the bearing sleeve. Taking a as the angle of gap's expansion towards the outside, an inner surface of the gap changing portion may be inclined at the angle α of 0° or greater. As is disclosed in the '833 patent, a gap inclination angle α of 0° indicates that it is acceptable to have a partial area of the gap changing portion being parallel to the rotating shaft.
In addition to having a gap widening seal portion provided on the sleeve, a shaft of the fluid dynamic bearing construction shown in FIG. 1 may be provided with a recess having an oil repellent film applied to its surface. Such recess prevents oil from splashing further out of the bearing gap, especially during the oil fill process, and causing damage. The conventional structure of the recess is more particularly shown in FIG. 3, where the conventional shaft recess is designated with a reference numeral 40. As shown in FIG. 3, the surface of the recess 40 forms a 90° angle with a shaft axis at the bottom and top portions of the recess (points A and B on FIG. 3). Accordingly, when oil is filled into the bearing gap and some of the oil is splashed into the shaft recess 40, some of the oil is collected at the bottom surface 40a, either at the edge or the corner of the recess, and has to be cleaned out by either wiping it out or vacuuming it out using a vacuum nozzle. However, it is difficult to remove the oil completely using either one of the above processes. Also, the necessity to remove the oil from the recess adds another step to the bearing assembly process and, therefore, increases the cost of the bearing.
Additionally, when an oil repellent film is applied to the surface of the recess 40, a caution must be exercised to prevent oil repellant from descending into the bearing gap filled with oil. Even a tiny amount of oil repellant can damage the fluid dynamic bearing assembly if the repellant descends into the bearing gap. Given the conventional shape of the recess, it is very difficult to apply the oil repellent film to the surface of the recess without dropping the repellant into the bearing gap. Also, when the repellant is applied to the conventional recess, drops of the repellant may collect in the recess pocket areas (points A and B in FIG. 3) and later slip into the bearing gap.
It is also difficult to apply the oil repellent film to the conventional recess evenly without missing any spots on the surface.